An EQA Scheme for TPMT Activity and Thiopurine Metabolites

Overview

Participation in EQA schemes, where available, is mandatory for the United Kingdom Accreditation Service (UKAS). No EQA scheme currently exists for TPMT and thiopurine metabolites, which is a potential shortcoming of these tests. A pilot of this project has been awarded funding by Clinical Pathology Accreditation (CPA). The purpose of this work is to collect samples for an EQA scheme for whole blood TPMT (activity and genotype) and thiopurine metabolites, which will be run in collaboration with UK NEQAS (United Kingdom National External Quality Assessment Service), and facilitate a comprehensive world-wide service that enables laboratories providing these tests to fulfil quality goals and ultimately provide optimal patient care.

The thiopurine S-methyl transferase (TPMT) enzyme is involved in the breakdown of thiopurine drugs (such as azathioprine), which are commonly used to treat inflammatory bowel disease and autoimmune diseases. Different individuals in the population have different, genetically determined, levels of TPMT. An individual with absent TPMT activity has a high risk of serious side effects from thiopurine drug treatment. In routine practice, TPMT activity in the blood is measured before starting patients on thiopurine drugs to determine an effective and safe starting dose. Thiopurine metabolites are the breakdown products of thiopurine drugs and are monitored in the blood of patients on thiopurine drugs to optimise their drug dose. These tests are performed by laboratories across Britain and around the world. It is important for patient care that the results are of high quality and consistent across different centres. One way of assessing this is through an EQA scheme. No EQA scheme currently exists for TPMT/thiopurine metabolites, which is a potential shortcoming of the tests. Venous blood collected from NHS staff volunteers and SWBH outpatients, who have had TPMT measured as part of their routine care, will be distributed to participating laboratories every two months. Where it is not possible to find suitable blood donors, pooled surplus blood samples from the laboratory will be used instead. The laboratories will test the samples and return results to UK NEQAS for comparison. A report will be compiled allowing them to compare themselves with other centres and make changes where necessary.